It is already known to use combinations of polyhydroxyl compounds and organic compounds containing at least two intramolecular carboxylic acid anhydride groups per molecule, in particular copolymers containing succinic acid anhydride, as binders for lacquers and coating compounds.
Coating compositions consisting of hydroxyl functional polymers and an ester anhydride containing at least two anhydride groups per molecule, in which the ester anhydride is an oligomer of trimellitic anhydride, are described in FR-A-2 392 092. The disadvantages of these compositions are not only the very poor solubility of the anhydride component in the solvents conventionally used for lacquers but also the imcompatibility with hydroxyl functional polymers.
EP-A-48 128 describes a coating composition consisting of a hydroxyl component having at least two hydroxyl groups per molecule, an anhydride component having at least two anhydride groups per molecule and an amine component. The use of monomeric aminoalcohols such as triethanolamine as cross-linking reagents as described in the said specification has a very deleterious effect on the solvent resistance of the resulting lacquers if a sufficiently long pot life is required, since the latter can only be achieved at the expense of a low cross-linking density which results in lacquer films with little resistance to solvents. Even when relatively high molecular weight polyols are used, e.g. hydroxyfunctional polyacrylate resins, it is hardly possible to obtain a sufficiently long pot life of the systems for practical purposes.
The use of combinations of organic compounds, in particular those based on polymers containing at least two isocyanate groups per molecule and poylhydroxyl compounds, as binders for lacquers and coatings is described, for example, in DE-A-2 014 383, U.S. Pat. Nos. 4,219,632, 4,264,748, DE-A-3 004 527, EP-A-130 313 and EP-A-130 322. Coatings based on these combinations all have excellent properties as lacquers but these copolymers containing isocyanate groups have the disadvantage of insufficient stability in storage due to their high isocyanate functionality and their high molecular weight. Hydrolysis of the isocyanate groups to amino groups proceeds rapidly, especially in the presence of atmospheric moisture, and is followed by very rapid molecular weight degradation and viscosity increase to the point of gelling.